1. Field of the Invention
This invention relates generally to the field of metal alloys useful for metal plating a wide variety of articles, including printed circuit boards and other electronic packaging devices. In particular, the invention relates to additives and their use as a grain refiner in tin and tin alloy plating formulations. Furthermore, the present invention relates to additives which improve the stability of metal plating solutions, particularly copper-based systems.
2. Background
For many years, tin-lead alloy electroplating has been the process of choice in applications requiring attachment of electronic components to printed wiring boards by soldering or reflowing. Such tin-lead alloys have been used as solders and solderable finishes on components. The temperature at which the solder melts is highly important. For tin-lead alloys the temperature of 183xc2x0 C. (eutectic Snxe2x80x94Pb) matches very well the currently used materials for electronic parts. Much higher melting temperatures may result in deformations of the printed wiring board laminate, whereas distinctly reduced liquidus temperatures may lead to unwanted melting of the solder joint during other thermal operations of the assembly process.
Due to its toxic properties lead will likely be banned in electronic parts in the near future. Many tin-lead alternatives have been suggested, including pure tin, tin-silver, tin-copper, and tin-bismuth.
Electroplating articles with tin coatings is generally well known in the industry. Electroplating methods involve passing a current between two electrodes in a plating solution where one electrode is the article to be plated. A common plating solution would be an acid tin plating solution containing (1) a dissolved tin salt (such as tin alkylsulfonate), (2) an acidic electrolyte (such as alkylsulfonic acid) in an amount sufficient to impart conductivity to the bath and (3) additives (such as surfactants, brighteners, levelers and suppressants) to enhance the effectiveness and quality of plating. For example, U.S. Pat. No. 5,174,887 (Federman et al.) discloses a process for the high speed electroplating of tin having as a surfactant an alkylene oxide condensation product of an organic compound having at least one hydroxy group and 20 carbon atoms or less. The organic compounds include an aliphatic hydrocarbon of between 1 and 7 carbon atoms, an unsubstituted aromatic compound or an alkylated aromatic compound having 6 carbon atoms or less in the alkyl moiety.
It is also well known to electroplate articles with tin-alloys. For example, U.S. Pat. No. 6,176,996 (Moon) discloses certain tin-alloy electroplating baths containing a tin salt, a metal salt selected from zinc, cobalt, bismuth or copper salts, methanesulfonic acid, a conductive compound and a complexing agent.
A problem with tin electroplating baths is instability, i.e. sludge may form in the bath with time. Such sludge results from oxidation and subsequent precipitation of the metals in the bath. A number of additives have been proposed to prevent or reduce such sludge formation, however such additives adversely affect certain characteristics of the metal deposit, such as grain refinement.
It thus would be desirable to have new electroplating compositions. It would be particularly desirable to have new electroplating compositions that have increased stability, i.e. prevent or reduce sludge formation. It also would be particularly desirable to have new electroplating compositions which exhibit improved grain refinement.
We have now found new electroplating compositions that effectively plate a wide variety of articles, including printed circuit boards and other electronic packaging devices. Electroplating compositions of the invention exhibit enhanced grain refinement and stability. Using compositions of the invention, precipitation of metals from the plating solutions is eliminated or at least significantly reduced.
Compositions and methods of the invention are particularly useful for depositing solderable finishes on electronic components such as printed wiring boards, semiconductor packaging, lead frames and connectors, and for depositing solder bumps such as those used on integrated circuits. The present compositions also may be used in a wide variety of other electronic device applications, such as in the manufacture of integrated circuits, optoelectronic devices and the like.
Electroplating compositions of the invention are characterized in significant part by an additive that dually functions as a grain refiner and a stabilizer. Preferably, such additive comprises one or more non-aromatic compounds having xcfx80 electrons that can be delocalized. Examples of such compounds include an xcex1,xcex2 unsaturated system or other conjugated system that contains a proximate (e.g., within 1-2 carbons of a carbon-carbon double bond) electron-withdrawing group.
Cyclic systems that contain endocyclic conjugation are generally preferred. Particularly preferred are keto-enole systems. Even more preferred are cyclic keto-enole systems which contain a rigid arrangement of two oxygen donors, particularly where the enole functionality is endocylic.
Keto functionalities having a low pKa value also are particularly preferred systems for use as stabilizer additives in electroplating compositions of the invention.
Preferable concentrations for such present additive for use in electroplating compositions of the invention are between about 2 and about 10,000 mg per L, more preferably between about 50 and about 2000 mg per L, in acid plating solutions.
Preferably, the present additive concentration is maintained at such concentrations throughout the entire or at least substantial portion of a plating cycle. Such maintenance of the stabilizer additive concentrations entails regular addition of the stabilizer additive during a plating cycle as the stabilizer additive component is consumed. Additive concentrations and replenishment rates during a plating cycle can be readily determined by known methods, such as the CPVS method as disclosed in U.S. Pat. Nos. 5,252,196 and 5,223,118, both assigned to the Shipley Company, or by the cyclic voltammetric stripping (CVS) methods, or liquid chromatography.
In addition to such a grain refiner/stabilizer additive, the plating bath may optionally contain one or more reducing agents, grain refiners such as hydroxy aromatic compounds, wetting agents, brightening agents, compounds to extend the current density range, such as carboxylic acids, levelers and the like. Mixtures of additives may also be used in the present invention.
The invention also includes articles of manufacture, including electronic devices such as printed circuit boards, multi-chip modules, lead frames, connectors, semiconductor packaging, optoelectronic devices and the like that contain a deposit produced from a plating solution of the invention.
Other aspects of the invention are discussed infra.